As electrical appliances at home and offices are further computerized, standardization of interfaces are promoted for connecting electrical appliances through networks in order to realize data exchange, remote-control and office automation. IEEE 1394 serial interfaces and USB interfaces are well known as typical interfaces among those interfaces. For example, an IEEE 1394 serial interface uses many branch nodes and twist pair cables. This interface is characterized in that appliances can be hot plugged at various locations at home or in offices.